SPECTROCHEMISTRY OF SOLUTIONS .23. CHANGES OF ENTHALPY AND ENTROPY IN THE FORMATION OF CONTACT ION-PAIRS - A VIBRATIONAL SPECTROSCOPIC APPRAISAL USING THIOCYANATE AND AZIDE SOLUTIONS

The vibrational spectra of solutions have been analyzed to assess both qualitatively and quantitatively the changes in enthalpy and entropy for ion pair formation in solutions of LiNCS, Mg(NCS)2, and LiN, in liquid ammonia, dimethylformamide, dimethylsulphoxide and acetonitrile, Contrary to predictions both the DELTA-H(ass) and DELTA-S(ass) terms are all positive in the cases examined, indicating that the driving force in the ion association process derives from solvent-solute restructuring, and not the energy of the interaction between the cation and anion. This characteristic of contact ion pair formation is likely to be found to be applicable over a wide range of solvents. The following specific values of the thermodynamic parameters at 298 K have been obtained: LiNCS/DMF, DELTA-G = - 1.3 (1) kJ mol-1 DELTA-H(ass) = + 1.8 (5) kJ mol-1, DELTA-S(ass) + 10 (2) J mol-1 K-1; LiNCS/DMSO, DELTA-G = + 0.9 (2) kJ mol-1 DELTA-H(ass) = + 0.3 (3) kJ mol-1, Mg(NCS)2/DMF, DELTA-G(ass) = 4.0 (3) kJ mol-1, DELTA-H(ass) = + 15 (4) kJ mol-1, DELTA-S = + 64 (17) kJ mol-1; LiN3/DMSO, DELTA-G(ass) = - 2.5 (3) kJ mol-1, DELTA-H(ass) = + 4.9 (9) kJ mol-1, DELTA-S(ass) = approximately + 25 (10) J K-1 mol-1.